The Reaserch On Key Technologies Of Non-Orthogonal Cooperative Transmission In Wireless Communication Networks

The emerging technologies such as the 5G communication,Internet of Things,Artificial Intelligence and the Big Data are forging a word where everything is intelligent,sensible,and connected.In such application scenarios,massive terminals,devices and sensors are connected to the network through wireless technologies,which leads to an enormous explosion in quantities and network capacity.Applications such as mobile video and live broadcasting are spreading everywhere in the downtown and countryside,boosting the demand on ultra-high speed communications by a large magnitude.In addition,applications like unmanned vehicles post strict requirements on real-time performance.The current 4G technology is not capable of such services,especially regarding the demands on the spectrum utilization,network capacity,speed of data transfer,or allowance on delay.Thus,the 5G technology has been proposed as the solution and would be the cornerstone of such application scenarios.Some technologies,such as Cognitive Radio(CR)and Non-Orthogonal Multiple Access(NOMA),have been the research hotspot in recent years.If the Quality of Service(QoS)of the spectrum authorized users,also named primary users would not be compromised,CR technology grant spectrum unauthorized users,also named secondary users opportunities to access to spectrum of primary network and they can complete the transmission by sharing the spectrum of the primary users.Therefore,the system would have better spectrum utilization and the shortage of spectrum resources could be mitigated.As an innovated multiplexing technology,NOMA allows multiple users to share time,frequency,and spreading code resources by optimizing the allocation of power at the transmitter.Successive Interference Cancellation(SIC)technology is employed at the receivers to demodulate the signal,which promotes the utilization of the spectrum and reliability of the transmission.Relaying Aided Cooperative Transmission(RACT)expands the communication range and its reliability by selecting the best relays in the cooperative transmission,which would also reduce the required transmission power.Improvement in the transmission can be expected by combining the above technologies,including building the Cognitive Radio Non-Orthogonal Multiple Access(CR-NOMA)scenario while adopting optimized relays in the network,and has made CR-NOMA the focus of the main-stream engineering academia.However,certain difficulties have been posed in the application of CR-NOMA.Firstly,intra and inter-cell interferences in a 5G scenario could greatly degrade its transmission performance.Secondly,channel estimation errors are typically found in the communication system so information on channel state cannot be accurately obtained.Lastly,the channel quality of the secondary users may not be comprehensively sensed and could be affected by the dynamic interference from the primary users of system,which decreases the performance of the secondary system.The major significances of this dissertation can be reflected in the following aspects.Firstly,a cooperative multi-hop relaying transmission scheme is proposed for the CR scenario,together with a Joint Relay Selection Mechanism(JRSM)to optimize the selection of relay points that are randomly distributed in the network.This solution also adopts the maximal ratio combining(MRC)strategy to improve the performance of the receiving system.By analyzing the secondary transmission in the Rayleigh fading channel of CR scenario,the expressions of the received signal in each stage as well as of the outage probability of the secondary system are obtained.The multi-hop relay cooperative transmission scheme proposed in this dissertation adopts a more flexible relay selection mechanism based on interference cancellation and is then simulated and compared to traditional transmission scheme models.The result suggests the outage probability and the transmission power loss of the secondary system with dual power constraints are both reduced,and the QoS of the secondary system is improved.Secondly,a CR-NOMA multi-hop relaying aided transmission mechanism is proposed,where the performance of the secondary system is improved by a better channel sensing strategy.In this scenario,the imperfect channel state information of the secondary system and the dynamic interference from the primary users are fully considered,and the strategy of demodulation would be adjusted accordingly for better system performance.The closed form expression of the outage probability at the secondary receiver is obtained.Simulation results suggest that the performance of the secondary system has been improved by considering the dynamic interference from the primary users and the quantification of the channel estimation errors.Thirdly,RACT strategy is introduced to CR-NOMA scenario for better performance and larger transmission range,and technologies such as SIC and beam-forming are adopted to achieve optimal demodulation and signal recovery.The simulation suggests that,compared to the traditional mechanism,the system proposed in this dissertation has better continuity of secondary transmission,and its outage probability is significantly reduced.